Field of the Invention
This invention relates to a system and method for Fischer Tropsch gas to liquid hydrocarbon production; specifically it relates to a system and method for injecting Fischer Tropsch water into a Fischer Tropsch tail gas prior to recycling the Fischer Tropsch tail gas to a front end of a syngas preparation unit for reformed gas (also called “synthesis gas,” or “syngas”) production, as part of a Fischer Tropsch natural gas to liquid hydrocarbon process.
Background of the Invention
The Fischer-Tropsch (or “Fischer Tropsch,” “F-T” or “FT”) process (or synthesis or conversion) involves a set of chemical reactions that convert a mixture of carbon monoxide and hydrogen (known as reformed gas, synthesis gas, or “syngas”) into liquid hydrocarbons (called “liquid FT hydrocarbons” herein). The liquid FT hydrocarbons may include a wax (“FT wax”) that may be liquid when produced but becomes solid as it cools. The process was first developed by German chemists Franz Fischer and Hans Tropsch in the 1920's. The FT conversion is a catalytic and exothermic process. The FT process is utilized to produce petroleum substitutes, typically from carbon-containing energy sources such as coal, natural gas, biomass, or carbonaceous waste streams (such as municipal solid waste), such petroleum substitutes being suitable for use as synthetic fuels, waxes and/or lubrication oils. The carbon-containing energy source is first converted into a reformed gas, using a syngas preparation unit in a syngas conversion. Depending on the physical form of the carbon-containing energy source, syngas preparation may involve technologies such as steam methane reforming, gasification, carbon monoxide shift conversion, acid gas removal, gas cleaning and conditioning. These steps convert the carbon source to simple molecules, predominantly carbon monoxide and hydrogen, which are active ingredients of synthesis gas. Syngas also contains carbon dioxide, water vapor, methane, and nitrogen. Impurities deleterious to catalyst operation such as sulfur and nitrogen compounds are often present in trace amounts and are removed to very low concentrations, often as part of synthesis gas conditioning. Once the syngas is created and conditioned, the syngas is used as an input to an FT reactor having an FT catalyst to make the liquid FT hydrocarbons in a Fischer-Tropsch synthesis process. Depending on the type of FT reactor that is used, the FT conversion of the syngas to liquid FT hydrocarbons takes place under appropriate operating conditions.
Turning to the syngas conversion step, to create the syngas from a natural gas feedstock, for example, methane in the natural gas reacts with steam and/or oxygen in a syngas preparation unit to create syngas. The syngas comprises principally carbon monoxide, hydrogen, carbon dioxide, water vapor and unconverted methane. Some types of syngas preparation units use syngas catalysts (also called “reformer catalysts”), while others do not. When partial oxidation is used to produce the synthesis gas, the syngas typically contains more carbon monoxide and less hydrogen than is optimal and, consequently, the steam is added to the react with some of the carbon monoxide in a water-gas shift reaction. The water gas shift reaction can be described as:CO+H2O⇄H2+CO2  (1)
Thermodynamically, there is an equilibrium between the forward and the backward reactions. That equilibrium is determined by the concentration of the gases present.
Turning now to the FT conversion step, the Fischer-Tropsch (FT) reactions for the FT conversion of the syngas to the liquid FT hydrocarbons may be simplistically expressed as:(2n+1)H2+nCO→CnH2n+2+nH2O,  (2)where ‘n’ is a positive integer.
The FT reaction is performed in the presence of a catalyst, called a Fischer-Tropsch catalyst (“FT catalyst”). Unlike a reagent, a catalyst does not participate in the chemical reaction and is not consumed by the chemical reaction itself, but accelerates the chemical reaction. In addition, a catalyst may participate in multiple chemical transformations. Catalytic reactions have a lower rate-limiting free energy of activation than the corresponding un-catalyzed reaction, resulting in higher reaction rate at the same temperature. However, the mechanistic explanation of catalysis is complex. Catalysts may affect the reaction environment favorably, or bind to the reagents to polarize bonds, e.g. acid catalysts for reactions of carbonyl compounds, or form specific intermediates that are not produced naturally, such as osmate esters in osmium tetroxide-catalyzed dihydroxylation of alkenes, or cause lysis of reagents to reactive forms, such as atomic hydrogen in catalytic hydrogenation.
In addition to liquid FT hydrocarbons, Fischer-Tropsch synthesis also commonly produces gases (“Fischer-Tropsch tail gases” or “FT tail gases”) and water (“Fischer-Tropsch water” or “FT water”). The FT tail gases typically contain CO (carbon monoxide), CO2 (carbon dioxide, which may also be written informally as “CO2”), H2 (hydrogen), light hydrocarbon molecules, both saturated and unsaturated, typically ranging from C1 to C4, and a small amount of light oxygenated hydrocarbon molecules such as methanol. Typically, the FT tail gases are mixed in an FT facility's fuel gas system for use as fuel.
The FT water may also contain contaminants, such as dissolved hydrocarbons, oxygenates (alcohols, ketones, aldehydes and carboxylic acids) and other organic FT products. Typically, the FT water is treated in various ways to remove the contaminants and is properly disposed of.
FIG. 1 and FIG. 2 depict conventional systems. FIG. 1 depicts a simplified block diagram for a conventional Fischer Tropsch system, including a steam methane reformer configuration. Natural gas 102 and steam 104 enter a syngas preparation unit 130, which, in the example of FIG. 1 comprises a steam methane reformer (“SMR”). Alternate conventional syngas preparation units may include an autothermal reformer, a hybrid reformer, or a partial oxidation reformer. A flue gas 132 and a reformed gas (or “syngas”) 134 exit the SMR 130 via a first flowline and a second flowline (not numbered in FIG. 1 separately from the fluids therein) respectively. The reformed gas 134 typically includes hydrogen, carbon monoxide, carbon dioxide, water vapor, nitrogen and methane. The reformed gas 134 passes to a syngas conditioning unit 160, whereby the reformed gas 134 is cooled, a process condensate stream 162 is recovered, and the hydrogen and carbon monoxide ratios of the reformed gas 134 are adjusted if necessary. A conditioned reformed gas 165 is sent via a third flowline (not numbered separately from the fluids therein) to a Fischer-Tropsch (“FT”) synthesis reactor 170. Outputs for the FT reactor 170 include an FT tail gas 172 that may be sent to a fuel system (not depicted), an FT water stream 174 that may be sent to a treatment system (not depicted), and a liquid FT hydrocarbon stream 180.
FIG. 2 depicts a more detailed view of the conventional SMR 130 of FIG. 1 and some of its associated equipment. A fuel gas flowline 206 conveying a fuel gas passes through a first flow control regulator 208 to a first burner 209a and a second burner 209b of the SMR 130. A first combustion air flowline 211 passes combustion air through a forced draft fan 212. A second combustion air flowline 213 conveys the combustion air from the forced draft fan 212 to a combustion air heater 214, which heats the combustion air. The heated combustion air passes via a third combustion air flowline 215 to the first and second burners 209a, 209b, where the heated combustion air is mixed and combusted with the fuel gas.
Continuing to refer to FIG. 2, a first natural gas feed flowline 202 conveys a natural gas feed to a natural gas preheater 241, which heats the natural gas feed. The preheated natural gas feed is conveyed through a second natural gas feed flowline 227 to a mixed feed preheater coil 228, downstream of an intersection with a second flow control regulator 226, which injects steam into the natural gas feed (further described below) to form a mixed gas feed. The mixed feed preheater coil 228 heats the mixed gas feed. A mixed feed gas flowline 229 conveys the heated mixed gas feed from the mixed feed preheater coil 228 to an input (not separately depicted) of an SMR tube 210 containing a steam methane reformer catalyst (not separately depicted). Various appropriate steam methane reformer catalysts are commercially available, including but not limited to those offered by Clariant and Johnson-Mathey. Exposed to higher temperatures from the first and second burners 209a, 209b and to the steam methane reformer catalyst, the heated mixed gas feed becomes a reformed gas. A reformed gas flowline 231 conveys the reformed gas from an output (not separately depicted) of the SMR tube 210 to a reformed gas boiler 239.
Referring again to FIG. 2, a boiler feed water line 201 conveys a boiler feed water stream to a steam drum 216. A first water line 235 conveys water from the steam drum 216 to the reformed gas boiler 239. A steam-water mixture is returned from the reformed gas boiler 239 to the steam drum 216 via natural circulation through a mixture flowline 236. A second water line 217 conveys water from the steam drum 216 to a steam generator 218 that generates steam from the water. A first steam flowline 219 conveys the steam from the steam generator 218 to the steam drum 216. Steam leaves the steam drum 216 via a second steam flowline 220a, 220b. Part of the steam in the second steam flowline 220a may be diverted through a third steam flowline 221 connected to the second steam flowline 220a. (Upstream of connection to the third steam flowline 221, the second steam flowline is designated as 220a in FIG. 2, while downstream the second steam flowline is designated 220b.) The third steam flowline 221 may convey the diverted part of the steam to a turbine or to other parts of the plant. The second steam flowline 220b carries remaining steam, which was not diverted to the third steam flowline 221, to a steam superheater 223. The steam superheater 223 superheats the remaining steam to very high temperatures. For example, if the steam leaving the steam drum 216 in the second steam flowline 220a was at a temperature of about 450° F., then the steam superheater 223 may typically heat the remaining steam to a temperature of about 700° F. Superheated steam leaves the steam superheater 223 via a fourth steam flowline 224. The fourth steam flowline 224 is connected to a fifth steam flowline 261. The second flow control regulator 226 is positioned on the fifth steam flowline 261, downstream of its connection with the fourth steam flowline 224. Downstream of its connection with the fifth steam flowline 261, the fourth steam flowline 224 is connected to a third flow control regulator 225. The fifth steam flowline 261 feeds the a part of the superheated steam from the fourth steam flowline 224 into the second natural gas flowline 227 to be mixed with the natural gas in the second natural gas flowline 227, upstream of the mixed feed preheater coil 228. The second and third flow control regulators 226, 225 may be adjusted to allow a predetermined amount of the superheated steam into the second natural gas flowline 227. Thus, a mixture of steam and natural gas are conveyed as the mixed feed gas in the mixed feed gas flowline 229 from the mixed feed preheater coil 228 to the input of the SMR tube 210.
Referring again to FIG. 2, when the reformed gas in the reformed gas flowline 231 has exited the SMR tube 210, the reformed gas may be at very high temperatures. A temperature of about 1600° F. for the reformed gas might be typical. The reformed gas flowline 231 conveys the reformed gas to the reformed gas boiler 239, which can cool the reformed gas to a first lower temperature, as an example, down to 800° F. Such a temperature may still be considered hot. A second reformed gas flowline 240 conveys the reformed gas, at the first lower temperature, from the reformed gas boiler 239 to the natural gas preheater 241, where the first lower temperature of the reformed gas is used to heat the natural gas feed from the first natural gas feed flowline 202. The reformed gas then passes through a third reformed gas flowline 234 to optional further cooling and/or treatment and to the FT reactor (not depicted in FIG. 2). A flue gas exits the SMR 130 via a flue gas flowline 232, which carries the flue gas to an induced draft fan 233 and from the induced draft fan 233 to a flue gas stack 237.
In the conventional SMR 130 of FIGS. 1 and 2, the FT tail gas may be mixed in a facility's fuel gas system for use as fuel. The FT water may contain contaminants, such as dissolved hydrocarbons, oxygenates (alcohols, ketones, aldehydes and carboxylic acids) and other organic FT products. Typically, the FT water is treated in various ways to remove the contaminants and is properly disposed of.
U.S. Pat. No. 7,323,497 B2 by Abbott et al. (“Abbott”), incorporated in its entirety herein by reference for all purposes not contrary to this disclosure, describes an alternative to the conventional process described above with respect to FIGS. 1 and 2. Abbott includes the step of feeding “co-produced water” [FT water] “to a saturator wherein it is contacted with hydrocarbon feedstock to provide at least part of the mixture of hydrocarbon feedstock and steam subjected to steam reforming.” (Abstract. See also Col. 10, lines 14-17.) However, while saturators are efficient, they may be expensive. In addition, saturators generally require a blow-down, the results of which must be properly disposed of. Moreover, using a saturator, the heated FT water in the saturator has a long residence time, which may result in unwanted side reactions among impurities producing heavy by-products. Abbott also discloses at least a two-stage reforming process. In the first stage, a partially reformed gas is produced through steam reforming. The steam reforming is performed after saturation of the feedstock with steam, the water for which may include FT water from the saturator. See Abbott, Column 4, lines 20-37. The steam reforming step may include “one or more (preferably one or two) stages of pre-reforming and/or primary steam reforming, to form a partially reformed gas.” (Abbott, Column 4, lines 45-49.) In a second stage, the partially reformed gas:                is then subjected to a step of partial combustion. The partially reformed gas fed to the partial combustion vessel may preferably additionally comprise a tail gas from the Fischer-Tropsch synthesis and/or, carbon dioxide recovered from the synthesis gas. Where primary and secondary reforming are used to produce the reformed gas stream it may also be desirable, in order to reduce the reforming duty on the primary reformer, to bypass a portion of the hydrocarbon (or hydrocarbon/steam mixture) around the primary reformer and feed it directly to the secondary reformer. In forming the feed stream for the partial combustion stage, the Fischer-Tropsch tail gas, and/or carbon dioxide and/or second hydrocarbon stream, may be combined separately in any order to the partially reformed gas or may be pre-mixed if desired before being fed to the partially reformed gas.(Abbott, Column 5, lines 19-34.) The partial combustion stage includes “combustion with a gas containing free oxygen supplied via burner apparatus.” Abbott, Column 5, lines 50-53. After combustion, “the hot partially combusted gas then passes through a bed of steam reforming catalyst to form the reformed gas mixture.” Abbott, Column 6, lines 25-27. Thus, in Abbott, the FT tail gas (and/or carbon dioxide and/or a second hydrocarbon) may be “added to the partially reformed gas before partial combustion thereof.” Abbott, Claim 7. In addition, Abbott indicates to “avoid the undesirable build up of inerts, it is desirable only to utilize tail gas recycle when the partial combustion step is performed using substantially pure oxygen.” Abbott, Column 8, lines 27-30. Sometimes, pure oxygen, as in the desirable embodiments disclosed by Abbott, is not readily available or is expensive to obtain. In addition, a single stage reformer might be preferred for some applications.        
Abbott further discloses, “Typically the de-watered synthesis gas contains 5 to 15% by volume of carbon dioxide (on a dry basis). In one embodiment of the invention, after separation of the condensed water, carbon dioxide may be separated from the de-watered synthesis gas prior to the Fischer-Tropsch synthesis stage and recycled to the synthesis gas production. Such recycle of carbon dioxide is preferred as it provides a means to control H2/CO ratio to achieve the optimal figure for FT synthesis of about 2.” (Abbott at Column 7, lines 5-13.)
U.S. Pat. No. 8,168,684 to Hildebrandt, et al. (“Hildebrandt”), incorporated in its entirety herein by reference for all purposes not contrary to this disclosure, discloses a Fischer Tropsch process with “CO2 rich syngas.” Hildebrandt defines a “CO2 rich syngas” as “a gas mixture in which there is CO2, H2 and CO. The CO2 composition in this mixture is in excess of the CO2 which would usually occur in conventional syngas.” (Hildebrandt, Column 2, lines 17-20.) The example described therein used coal as a feedstock. (See Hildebrandt at Col. 4, line 32: “The feed considered was coal.”) Hildebrandt also mentions the use of feedstocks comprising methane from natural gas (Hildebrandt at Col. 3, lines 36-40 and Col. 5, lines 23-25) and gas “generated by fermentation of natural waste dumps” (Hildebrandt at Col. 5, lines 23-25). Hildebrandt at Col. 2, lines 20-21 further states, “The CO2 is utilized as a reactant and is converted into the desired product.” Claim 1 of Hildebrandt recites in part the production of “hydrocarbons according to the overall process mass balance:CO2+3H2CH2+2H2O,″  (3)which is an equation known to work with iron-based FT catalysts, but not known to work with cobalt-based FT catalysts. See, for example, “Comparative study of Fischer-Tropsch synthesis with H2/CO and H2/CO2 syngas using Fe- and Co-based catalysts,” T. Riedel, M. Claeys, H. Schulz, G. Schaub, S. Nam, K. Jun, M. Choi, G. Kishan, K. Lee, in APPLIED CATALYSTS A: GENERAL 186 (1999), pp. 201-213 (“Riedel et al.”), which at page 212 concluded, “Fischer-Tropsch CO2 hydrogenation would be possible even in a commercial process with iron, however, not with cobalt catalysts.” Hildebrandt does not, however, disclose the FT catalyst or the type of FT catalyst used in the FT process(es) described.
Hildebrandt further notes, “Unreacted carbon dioxide, carbon monoxide and hydrogen may be recirculated from the Fischer Tropsch synthesis section (5) into the gasifier/reforming process stage (3) via a conduit (7) or back to the Fischer Tropsch synthesis section.” (Hildebrandt at Col. 3, lines 28-31.)
Accordingly, there are needs in the art for novel systems and methods for producing reformed gas or syngas. Desirably, such systems and methods enable using FT tail gas and FT water to make additional syngas without requiring a saturator or a source of pure oxygen.